Question: Tiffany is 21 years younger than Michael. Four years ago, Michael was 4 times older than Tiffany. How old is Michael now?
Explanation: We can use the given information to write down two equations that describe the ages of Michael and Tiffany. Let Michael's current age be $m$ and Tiffany's current age be $t$ The information in the first sentence can be expressed in the following equation: $m = t + 21$ Four years ago, Michael was $m - 4$ years old, and Tiffany was $t - 4$ years old. The information in the second sentence can be expressed in the following equation: $m - 4 = 4(t - 4)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $m$ , it might be easiest to solve our first equation for $t$ and substitute it into our second equation. Solving our first equation for $t$ , we get: $t = m - 21$ . Substituting this into our second equation, we get the equation: $m - 4 = 4($ $(m - 21)$ $ -$ $ 4)$ which combines the information about $m$ from both of our original equations. Simplifying the right side of this equation, we get: $m - 4 = 4m - 100$ Solving for $m$ , we get: $3 m = 96$ $m = 32$.